Fluorescence intensity and UV absorption changes accompanying dissociation and association of regulatory light chain of scallop adductor myosin

Dissociation and association of regulatory light chains of scallop myosin were found to be accompanied by changes in the fluorescence intensity and in the UV absorption spectrum. The changes in the two optical properties of scallop myosin and the dissociation and association of regulatory light chains were studied as a function of the magnesium and calcium concentrations. The results thus obtained suggested that there are two different types of attachment between regulatory light chains and "desensitized" myosin; one type is a calcium-specific attachment, and the other type of attachment can be mediated by either calcium or magnesium ions. These changes in the optical properties of scallop myosin were distinguishable from those induced by Mg-ATP; for example, with "desensitized" scallop myosin, the former changes were not observed but the latter were.     

Proximity of regulatory light chains in scallop myosin

The distance between the regulatory light chains of the two heads of the scallop myosin molecule was estimated with the aid of two photolabile cross-linkers, benzophenone maleimide and p-azidophenacylbromide. These cross-linkers selectively alkylate thiol groups and have a maximum length of about 9 A. One of the two regulatory light chains of scallop myosin was removed by treatment of myofibrils at 10 degrees C with EDTA and replaced with a foreign regulatory light chain carrying a cross-linker. Cross-linking between the scallop and foreign regulatory light chains was effected by photolysis. This was demonstrated by incubating nitrocellulose transfers of sodium dodecyl sulfate/polyacrylamide gels of the photolyzed hybrid myofibrils with specific antibodies against the different light chains, followed by fluorescein isothiocyanate-125I-labeled secondary antibody. Scallop regulatory light chains cross-linked extensively (20 to 50%) with Mercenaria regulatory light chains (cysteine in position approximately 50) in solutions that induce rigor in skinned fibers (no ATP) and in relaxing solutions (ATP but no Ca2+). Neither the regulatory light chains of chicken skeletal myosin (cysteines 129 and 157) nor those of gizzard myosin (cysteine 108) were cross-linked to scallop regulatory light chains in either medium. These results indicate that the N-terminal portions of the myosin regulatory light chains can approach each other within 9 A or less, while the distance between the C-terminal halves exceeds 9 A, and support the view that the N termini of the regulatory light chains point toward the myosin rod. Since the relative distance between the regulatory light chains of the two myosin heads is not altered between rigor and rest, we suggest that motion of the essential light chains is mainly responsible for the observed difference in the relative positions of the regulatory and essential light chains between conditions of rigor and rest.     

The mechanism of regulatory light chain dissociation from scallop myosin.

The dissociation of the regulatory light chains from scallop myosin subfragments, on addition of EDTA, was investigated by using the fluorophore 8-anilinonaphthalene-1-sulphonate as a probe. The rate of this process (0.014 s-1) was partially limited by the rate of Mg2+ dissociation (0.058 s-1) from the non-specific high-affinity site. The dissociation of the regulatory light chain subfragment 1 was less extensive than from heavy meromyosin. Reassociation of the scallop regulatory light chain was induced on addition of Mg2+, but it appeared to be limited by a first-order step. The nature of this step was revealed by the kinetics of Mercenaria regulatory light chain association. Scallop heavy meromyosin, denuded of its regulatory light chains, exists in a refractory state, whose reversal to the nascent state limits the rate of light chain association (0.006 s-1). The formation of the refractory state is the driving force for the net dissociation of regulatory light chains from scallop heavy meromyosin. This mechanism is discussed with reference to existing structural information on light-chain-denuded myosin.     

Regulation of scallop myosin by mutant regulatory light chains

Scallop adductor myosin is regulated by its subunits; the regulatory light chain (R-LC) and essential light chain (E-LC). Myosin light chains suppress muscle activity in the absence of calcium and are responsible for relaxation. The binding of Ca2+ to the myosin triggers contraction by releasing the inhibition imposed on myosin by the light chains. To map the functional domains of the R-LC, we have carried out mutagenesis followed by bacterial expression. Both wild-type and mutant proteins were hybridized to scallop myosin heavy chain/E-LC to map the regions of the light chain that are responsible for the binding to the myosin heavy chain/E-LC, for restoring the specific calcium-binding site, and controlling the myosin ATPase activity. The R-LC is expressed in Escherichia coli using the pKK223-3 (Pharmacia) expression vector and has been purified to greater than 90% purity. E. coli-expressed wild-type R-LC differs from the native R-LC by having the initiating methionine residue and an unblocked NH2 terminus. The wild-type R-LC restores Ca2+ binding and Ca2+ sensitivity when hybridized to scallop myosin. A point mutation of the sixth Ca2(+)-liganding position of domain I (Asp39----Ala39) results in a R-LC that binds more weakly to the heavy chain/E-LC and restores the specific Ca2(+)-binding site but not regulation of the actin-activated Mg2+ ATPase. A second mutation was produced by substituting the last 11 residues of the COOH terminus with 15 different residues. This mutant restores the specific Ca2(+)-binding site, but does not restore Ca2+ regulation to the actin-activated ATPase activity. Several other point mutations do not alter light chain function. The experiments directly establish that the divalent cation-binding site of domain I is functionally distinct from the specific Ca2(+)-binding site. The results indicate that an intact domain I and the COOH terminus are required to suppress the myosin ATPase activity. The fact that the domain I mutation and the COOH-terminal mutation disrupt regulation but do not affect Ca2(+)-binding indicates that these two aspects of regulation are separable and, therefore, the R-LC has distinct functional regions.     

The light chains of scallop myosin as regulatory subunits

In molluscan muscles contraction is regulated by the interaction of calcium with myosin. The calcium dependence of the aotin-activated ATPase activity of scallop myosin requires the presence of a specific light chain. This light chain is released from myosin by EDTA treatment (EDTA-light chains) and its removal desensitizes the myosin, i.e. abolishes the calcium requirement for the actin-activated ATPase activity, and reduces the amount of calcium the myosin binds; the isolated light chain, however, does not bind calcium and has no ATPase activity. Calcium regulation and calcium binding is restored when the EDTA-light chain is recombined with desensitized myosin preparations. Dissociation of the EDTA-light chain from myosin depends on the concentration of divalent cations; half dissociation is reached at about 10^?5 M-magnesium or 10^?7 M-calcium concentrations. The EDTA-light chain and the residual myosin are fairly stable and the components may be kept separated for a day or so before recombination.Additional light chains containing half cystine residues (SH-light chains) are detached from desensitized myosin by sodium dodecyl sulfate. The EDTA-light chains and the SH-light chains have a similar chain weight of about 18,000 daltons; however, they differ in several amino acid residues and the EDTA-light chains contain no half cystine. The SH-light chains and EDTA-light chains have different tryptic fingerprints. Both light chains can be prepared from washed myofibrils.Densitometry of dodecyl sulfate gel electrophoresis bands and Sephadex chromatography in sodium dodecyl sulfate indicate that there are three moles of light chains in a mole of purified myosin, but only two in myosin treated with EDTA. The ratio of the SH-light chains to EDTA-light chains was found to be two to one in experiments where the total light-chain complements of myosin or myofibril preparations were carboxymethylated. A similar ratio was obtained from the densitometry of urea-acrylamide gel electrophoresis bands. We conclude that a myosin molecule contains two moles of SH-light chain and one mole of EDTA-light chain, and that the removal of a single EDTA-light chain completely desensitizes scallop myosin.Heavy meromyosin and S-1 subfragment can be prepared from scallop myosin. Both of these preparations bind calcium and contain light chains in significant amounts. The heavy meromyosin of scallop is extensively degraded; the S-1 preparation, however, is remarkably intact. Significantly, heavy meromyosin has a calcium-dependent actin-activated ATPase while the S-1 does not require calcium and shows high ATPase activity in its absence. These results suggest that regulation involves a co-operativity between the two globular ends of the myosin.Desensitized scallop myosin and scallop S-1 preparations can be made calcium sensitive when mixed with rabbit actin containing the rabbit regulatory proteins. This result makes it unlikely that specific light chains of myosin are involved in the regulation of the vertebrate system.The fundamental similarity in the contractile regulation of molluscs and vertebrates is that interaction between actin and myosin in both systems requires a critical level of calcium. We propose that the difference in regulation of these systems is that the interaction between myosin and actin is prevented by blocking sites on actin in the case of vertebrate muscles, whereas in the case of molluscan muscles it is the sites on myosin which are blocked in the absence of calcium.     

Structural changes that occur in scallop myosin filaments upon activation

Myosin filaments isolated from scallop striated muscle have been activated by calcium-containing solutions, and their structure has been examined by electron microscopy after negative staining. The orderly helical arrangement of myosin projections characteristic of the relaxed state is largely lost upon activation. The oblique striping that arises from alignment of elongated projections along the long-pitched helical tracks is greatly weakened, although a 145 A axial periodicity is sometimes partially retained. The edges of the filaments become rough, and the myosin heads move outwards as their helical arrangement becomes disordered. Crossbridges at various angles appear to link thick and thin filaments after activation. The transition from order to disorder is reversible and occurs over a narrow range of free calcium concentration near pCa 5.7. Removal of nucleotide, as well as dissociation of regulatory light chains, also disrupts the ordered helical arrangement of projections. We suggest that the relaxed arrangement of the projections is probably maintained by intermolecular interactions between myosin molecules, which depend on the regulatory light chains. Calcium binding changes the interactions between light chains and the rest of the head, activating the myosin molecule. Intermolecular contacts between molecules may thus be altered and may propagate activation cooperatively throughout the thick filament.     

Effect of regulatory light chain on chymotryptic digestion of scallop adductor myosin

Chymotryptic digestability of scallop myosin was studied by measuring (a) changes in the gel electrophoretic pattern and (b) production of the soluble fraction obtained by centrifugation. Chymotryptic digestion of essential light chain (SH-LC) was strongly inhibited by association of regulatory light chain (R-LC) with myosin. This is in agreement with the observation of Stafford et al. (Biochemistry 18, 5273 (1979]. SH-LC and R-LC were both more resistant to the chymotryptic digestion when R-LCs were associated with myosin in the presence of calcium than when they dissociated from myosin in the presence of EDTA. In contrast, heavy chains of scallop myosin were digested more quickly in the presence of calcium than EDTA. This suggests that association of R-LC induces reversible changes in the heavy chain conformation, which lead to an increase in the chymotryptic digestability of heavy chains. The chymotryptic digestability of scallop myosin increased in two distinct phases as the calcium concentration in the digestion medium was increased, but monophasically as the magnesium concentration was increased. The magnesium increased the digestability by approximately half as much as did calcium. These findings suggest two types of attachment between regulatory light chains and desensitized myosin: one mediated specifically by low concentrations of calcium ions, the second by higher concentrations of either calcium or magnesium.     

Crystal structure of a phosphorylated light chain domain of scallop smooth-muscle myosin

We have determined the crystal structure of a phosphorylated smooth-muscle myosin light chain domain (LCD). This reconstituted LCD is of a sea scallop catch muscle myosin with its phosphorylatable regulatory light chain (RLC SmoA). In the crystal structure, Arg¹⁶, an arginine residue that is present in this isoform but not in vertebrate smooth-muscle RLC, stabilizes the phosphorylation site. This arginine interacts with the carbonyl group of the phosphorylation-site serine in the unphosphorylated LCD (determined previously), and with the phosphate group when the serine is phosphorylated. However, the overall conformation of the LCD is essentially unchanged upon phosphorylation. This result provides additional evidence that phosphorylation of the RLC is unlikely to act as an on-switch in regulation of scallop catch muscle myosin.     

Akazara scallop myosins hybridized with DTNB-light chains of skeletal myosin and with regulatory light chains of gizzard myosin

Two different hybrid myosins were obtained by combining "desensitized" myosin (DM) of Akazara scallop striated adductor with rabbit skeletal DTNB-light chains (DTNB-LC) and with chicken gizzard regulatory light chains (GR-LC). Using the two hybrid myosins, the following were found: (a) DTNB-LC has an inhibitory effect on the Mg-ATPase activities of Akazara DM and acto-DM both in the absence of calcium and in its presence. (b) DTNB-LC also has an enhancing effect on the superprecipitation activity of acto-DM. (c) The Mg-ATPase activities of DM and acto-DM are made sensitive to calcium by GR-LC, regardless of whether GR-LC is phosphorylated or unphosphorylated. (d) However, the Mg-ATPase activity of acto-myosin hybridized with phosphorylated GR-LC is definitely higher than that of acto-myosin hybridized with unphosphorylated GR-LC.     

A cAMP-dependent regulatory protein for RLC-a myosin kinase catalyzing the phosphorylation of scallop smooth muscle myosin light chain

A cAMP-dependent regulatory protein which modulates the phosphorylation of scallop myosin regulatory light chain-a (RLC-a) by RLC-a myosin kinase (aMK) (Sohma, H. & Morita, F. (1986) J. Biochem. 100, 1155-1163) was purified from the scallop smooth muscle. RLC-a is abundant in the opaque portion of scallop smooth muscle, one of the catch muscles. The regulatory protein for aMK was purified by employing successively DEAE Toyopearl ion exchange chromatography, Sepharose 4B-8(6-aminohexylamino)cAMP affinity chromatography, and Sephadex G 100 gel filtration. The molecular mass of the regulatory protein was 41 kDa, based on the mobility in polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. With increasing amounts of the regulatory protein, the aMK activity decreased, and complete inhibition was observed at the concentration of twice that of aMK. The aMK activity inhibited by the regulatory protein was restored by the addition of cAMP. These results suggest that aMK is similar to a catalytic subunit of cAMP-dependent protein kinase, and the protein reported here is similar to its regulatory subunit. aMK may exist as an inactive form, as a combination with this regulatory protein, in vivo and be deinhibited by an increase in the intracellular concentration of cAMP. We discuss a possible correlation between the phosphorylation of RLC-a in myosin catalyzed by aMK and the catch state of the opaque portion of scallop smooth muscle.     

Helical reconstruction of frozen-hydrated scallop myosin filaments.

Native myosin filaments from scallop striated muscle that have been rapidly frozen in relaxing solutions appear to be well preserved in vitreous ice. Electron micrographs of samples at -177 degrees C were recorded with an electron dose of 10 e/A2 at 1.5 microns defocus. After filament images were straightened by spline-fitting, several transforms showed well-defined layer-lines arising from the helical structure of the filament. A set of 17 near-meridional layer-lines has been collected and corrected for background and for phase and amplitude contrast functions. Preliminary helical reconstructions from this still incomplete data set reveal aspects of structure that were not apparent from earlier analysis of negatively stained filaments from scallop muscle. Individual pear-shaped myosin heads now appear to be well resolved from each other and from the filament backbone. The two heads of each myosin molecule appear to be splayed apart axially. The reconstructions also reveal that the filament backbone has a polygonal shape in cross-section, and that it appears to contain seven peripherally located subfilaments.     

The regulatory domain of the myosin head behaves as a rigid lever.

The regulatory domain of the myosin head is believed to serve as a lever arm that amplifies force generated in the catalytic domain and transmits this strain to the thick filament. The lever arm itself either can be passive or may have a more active role storing some of the energy created by hydrolysis of ATP. A structural correlate which might distinguish between these two possibilities (a passive or an active role) is the stiffness of the domain in question. To this effect we have examined the motion of the proximal (ELC) and distal (RLC) subdomains of the regulatory domain in reconstituted myosin filaments. Each subdomain was labeled with a spin label at a unique cysteine residue, Cys-136 of ELC or Cys-154 of mutant RLC, and its mobility was determined using saturation transfer electron paramagnetic resonance spectroscopy. The mobility of the two domains was similar; the effective correlation time (tau(eff)) for ELC was 17 micros and that for RLC was 22 micros. Additionally, following a 2-fold change of the global dynamics of the myosin head, effected by decreasing the interactions with the filament surface (or the other myosin head), the coupling of the intradomain dynamics remained unchanged. These data suggest that the regulatory domain of the myosin head acts as a single mechanically rigid body, consistent with the regulatory domain serving as a passive lever.     

Regulatory and essential light-chain interactions in scallop myosin. I. Protection of essential light-chain thiol groups by regulatory light-chains

The reactivity of the thiol groups of the essential light-chains of scallop myosin is greatly reduced by the presence of regulatory light-chains on myosin. The thiol groups of the essential light-chains react with iodoacetate only if the regulatory light-chains have been removed by treatment with EDTA. No alkylation of the essential light-chains could be detected in myosins containing regulatory light-chains (untreated or reconstituted myosins) after an overnight incubation with excess iodoacetate at 4 °C. In contrast, similar treatment alkylated two to three thiol groups of essential light-chains in desensitized myosins from which the regulatory light-chains had been removed. In addition, up to seven of the 20 heavy-chain thiols were also alkylated; however, the reactivity of the heavy-chain thiols did not depend on the presence of the regulatory light-chains. ATPase activities were not inhibited by alkylation with iodoacetate. Regulatory light-chains also protected essential light-chain thiols against reaction with N-iodoacetyl-N^′-(l-sulfo-5-naphthyl) ethylenediamine and against dansylation at pH 6.7, although treatment with these reagents caused a considerable loss of ATPase activities. Rebinding of the regulatory light-chains was impaired by alkylation. The results indicate an extensive interaction between the regulatory and the essential light-chains in scallop myosin.     

Calcium binding and conformation of regulatory light chains of smooth muscle myosin of scallop

Calcium binding was studied with two regulatory light chains (RLC-a and RLC-b) of smooth muscle myosin of scallop. With the equilibrium dialysis method, the binding of 0.98 mol Ca2+ per mol of RLC-b was observed with a dissociation constant of 2.3 X 10(-5) M. Similar values for RLC-b, 1.9 X 10(-5) M, and RLC-a, 1.5 X 10(-5) M, were obtained by measuring the difference absorption spectrum induced by Ca2+. The difference molar absorption coefficient at 288 nm was 159 and 209 M-1 X cm-1 for RLC-a and RLC-b, respectively, while it was -34 M-1 X cm-1 for the regulatory light chain of striated muscle myosin of scallop (RLC-st). Proton NMR spectra of the three light chains were very similar to each other and were broader than those of other Ca2+ binding proteins, parvalbumin and calmodulin. The regulatory light chains may be more rigid than in these Ca2+ binding proteins. CD spectra were measured for the three light chains, and the estimated helix contents were 27, 29, and 24%, respectively, for RLC-a, RLC-b, and RLC-st. All these results in comparison with the primary structures led us to suppose that the polypeptide of regulatory light chains is folded in such a way that domain 4 becomes near to the calcium binding site of domain 1. The decrease in intact light chains on trypsin digestion was determined for the gel electrophoretic patterns. RLC-a was 6 times more susceptible to the tryptic digestion than RLC-b.(ABSTRACT TRUNCATED AT 250 WORDS)     

An enzyme-probe method to detect structural changes in the myosin rod

The temperature-dependence of local melting within the alpha-helical, coiled-coil structure of rabbit myosin rod has been investigated by following changes in the rate constants of proteolytic digestion. The kinetics of fragmentation of the rod by three different enzymes (alpha-chymotrypsin, trypsin and papain) over the temperature range 5 to 40 degrees C (pH 7, I = 0.5) has been monitored by electrophoresis of the digestion products on sodium dodecyl sulfate/polyacrylamide gels. All rate constants were corrected for the intrinsic temperature-dependence of the enzyme by comparison with model substrates. Results from the three enzyme-probes are similar in showing that local melting within the rod occurs in two distinct stages. At temperatures between 5 and 25 degrees C, melting is confined to a restricted segment of the rod structure near the light meromyosin/heavy meromyosin junction. At temperatures between 25 and 40 degrees C, a wider segment of the rod lysing between the junction and the short subfragment-2 segment (the hinge domain) appears to be melting, judging from the broad spectrum of cleavage sites observed in this region. Results are compared with those from other physicochemical methods that measure the hinging or opening of the coiled-coil structure of the rod.     

Electron microscopy of negatively stained scallop myosin molecules. Effect of regulatory light chain removal on head structure

The heads of myosin molecules from the striated adductor muscle of scallop have been studied by electron microscopy after negative staining. In common with vertebrate skeletal muscle myosin visualized by this method, the scallop myosin heads were pear-shaped and often showed pronounced curvature. Staining suggestive of two or, more frequently, three domains could often be observed. Removal of regulatory light chains (R-LCs) resulted in a reduction in the length of the heads of about 2.6 nm, with no significant change in maximum width. In desensitized preparations a majority of heads displayed anticlockwise curvature, whereas intact heads were usually seen curved clockwise. Analysis of the head curvature in both intact and desensitized molecules was consistent with an ability of each head to rotate about its long axis. Desensitization resulted in an increased incidence of heads showing two domains. It seems likely that the reduction in length upon removal of the R-LC is due to the two small domains located in the neck region of the head collapsing into one.     

Ca2(+)-dependent protein phosphatase which dephosphorylates regulatory light chain-a in scallop smooth muscle myosin

Ca2(+)-dependent protein phosphatase was purified from scallop adductor smooth muscle by a combination of DEAE-Toyoperal 650S ion exchange chromatographies and gel filtration on Sephacryl S-300. The phosphatase consisted of two subunits having molecular weights of 60 and 19 kDa. Phosphorylated regulatory light chain-a (RLC-a) was dephosphorylated by this phosphatase both in free and bound states in myosin prepared from the opaque portion of scallop smooth muscle (opaque myosin). The dephosphorylation was activated by Ca2+. The half maximal activation was a 1 microM free Ca2+ in the presence of calmodulin and 7 microM free Ca2+ in the absence of calmodulin. Opaque myosin phosphorylated at the heavy chain was not dephosphorylated with this phosphatase. p-Nitrophenyl phosphate was dephosphorylated. In addition to Ca2+, the phosphatase activity for RLC-a was activated by Mn2+, while p-nitrophenylphosphatase activity was activated by Mg2+ more strongly than by Mn2+. The pH-activity curves showed a maximum at pH 7 in the presence of Mn2+, but at around pH 8 in the presence of Mg2+. This phosphatase is similar to phosphatase 2B or calcineurin. The possible regulatory function of this phosphatase in scallop catch muscle is discussed.     

Regulatory properties of single-headed fragments of scallop myosin.

Calcium control was studied in single-headed myosin and subfragment-1 (S1) preparations obtained by papain digestion of scallop myosin. Single-headed myosin, containing light chains in stoichiometric amounts, was calcium regulated; in contrast, the actin-activated Mg-ATPase of all S1 species lacked calcium sensitivity. Both regulatory and essential light chains were retained by S1 and single-headed myosin preparations provided divalent cations were present during papain digestion, although a peptide amounting to 10% of the mass was removed from regulatory light chains. The modified regulatory light chain retained its ability to confer calcium binding and restore calcium sensitivity to the ATPase of desensitized myofibrils. Regulatory light chains protected the essential light chains from fragmentation by papain. S1 bound regulatory light chains with a uniformly high affinity and appeared to consist of a single species. The results demonstrate that head to head interactions are not obligatory for calcium control, although they may occur in the intact myosin molecule, and suggest a role for the subfragment-2 region in calcium regulation of myosin.